Sputtering as a means to deposit thin films of a desired material on a substrate has become important in the manufacture of semiconductor devices such as integrated circuits. In a sputtering system, material to be deposited on a substrate is removed from a sputter target by bombardment of the target with ions. However, ion bombardment not only causes atoms or molecules of the target material to be ejected from the target, it also imparts thermal energy to the target.
Normally, a sputtering system comprises a sputter source, a vacuum chamber, and means for positioning and holding the substrate in proximity to the sputter source. The sputter source normally comprises a target from which material is to be sputtered, means for holding the target in place, means for creating a plasma in the vicinity of the target, means for causing ions in the plasma to bombard the target, and means for cooling the target to prevent overheating.
Various means have been used in the past for holding sputter targets in place within the sputter sources. Such holding means must insure that the target maintains good thermal contact with the cooling means. In a vacuum environment this generally requires good physical contact. On the other hand, sputter targets must be replaced from time to time as the target material is removed and the target has eroded away. Moreover, in certain research applications there is the need for frequent target changes so that experiments can proceed with different materials.
The time required to replace the sputter target and the method of replacement are often significant considerations to users of sputtering systems. For example, the semiconductor manufacturing industry is particularly sensitive to equipment down time because of the disruptive and costly effects it can have on an entire semiconductor fabrication line. Similarly, the method of target replacement should minimize the introduction of foreign material into the sputtering system vacuum chamber to avoid possible contamination problems with the substrate.
In some prior art sputter sources, the sputter target is physically soldered to a cooling plate. While this insures good thermal contact, it makes replacement of the target a difficult and time consuming problem. In other designs, targets are physically bolted to a cooling means. While this makes target replacement somewhat easier, the thermal contact is not as good.
Other sputter sources have annular targets where cooling is accomplished by thermal contact between an outer rim of the target and a peripheral cooling wall. In such sources, good thermal contact is maintained between the target rim and the cooling wall by thermal expansion of the target. Examples of such sources are shown in U.S. Pat. Nos. 4,100,055; 4,385,979; 4,457,825 and 4,657,654.
In a more recent design, for example, as shown in U.S. Pat. No. 4,855,033 a sputtering target cathode includes a cathode nest which receives the target. The cathode nest includes internal helical threads on its inner peripheral surface, while the target includes helical threads on its outer periphery which match the threads in the target nest. The target includes a central aperture formed by a sloped surface which receives a retainer. Screws are insertable through apertures in the retainer to retain a target within the target nest.
As mentioned earlier, the ion bombardment imparts thermal energy to the target causing the target to thermally expand. In a situation where the target is internally threaded into the target nest, for example as shown in U.S. Pat. No. 4,855,033, the thermal expansion of the target makes it quite difficult for the removal of the target, even after the target has cooled.
Prior methods of removing targets include using such tools as a hammer and chisel and tapping at the target in an opposite sense to "unscrew" the target out of its complementary target nest. This method has proven to be very time consuming in the changeover of targets, and can also introduce foreign material into the vacuum chamber causing possible contamination problems with the substrate. Moreover, damage to the target nest or other expensive sputtering equipment may occur.
Accordingly, it is an object of the invention to make it possible to quickly and easily change sputter targets in a sputter source of the type where the sputtering target is of the type having helical threads on the outer periphery of the target.
Another object of the invention is to provide a quick change sputter target and source structure which minimizes the risk of contamination of the sputter system vacuum chamber and damage to the system equipment.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.